Chapter 12 Coordination Chemistry IV

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Chapter 12Coordination Chemistry IV

• Reactions and Mechanisms

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Coordination Compound Reactions

• Goal is to understand reaction mechanisms
• Primarily substitution reactions, most are rapid
  Cu(H2O)62 4 NH3 ? Cu(NH3)4(H2O)22 4
  H2Obut some are slowCo(NH3)63 6 H3O ?
  Co(H2O)63 6 NH4

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Coordination Compound Reactions

• Labile compounds - rapid ligand exchange
  (reaction half-life of 1 min or less)
• Inert compounds - slower reactions
• Labile/inert labels do not imply
  stability/instability (inert compounds can be
  thermodynamically unstable) - these are kinetic
  effects
• In general
• Inert octahedral d3, low spin d4 - d6, strong
  field d8 square planar
• Intermediate weak field d8
• Labile d1, d2, high spin d4 - d6, d7, d9, d10

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Substitution Mechanisms

• Two extremesDissociative (D, low coordination
  number intermediate)Associative (A, high
  coordination number intermediate)
• SN1 or SN2 at the extreme limit
• Interchange - incoming ligand participates in the
  reaction, but no detectable intermediate
• Can have associative (Ia) or dissociative (Id)
  characteristics
• Reactions typically run under conditions of
  excess incoming ligand
• Well look briefly at rate laws (details in
  text), consider primarily octahedral complexes

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Substitution Mechanisms
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Substitution Mechanisms
Pictures
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Substitution Mechanisms
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Determining mechanisms

• What things would you do to determine the
  mechanism?

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Dissociation (D) Mechanism

• ML5X ? ML5 X k1, k-1ML5 Y ? ML5Y k2
• 1st step is ligand dissociation. Steady-state
  hypothesis assumes small ML5, intermediate is
  consumed as fast as it is formed
• Rate law suggests intermediate must be observable
  - no examples known where it can be detected and
  measured
• Thus, dissociation mechanisms are rare -
  reactions are more likely to follow an
  interchange-dissociative mechanism

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Interchange Mechanism

• ML5X Y ? ML5X.Y k1, k1 ML5X.Y ? ML5Y
  X k2 RDS
• 1st reaction is a rapid equilibrium between
  ligand and complex to form ion pair or loosely
  bonded complex (not a high coordination number).
  The second step is slow.Reactions typically
  run under conditions where Y gtgt ML5X

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Interchange Mechanism

• Reactions typically run under conditions where
  Y gtgt ML5X M0 ML5X ML5X.Y Y0 ?
  Y
• Both D and I have similar rate laws
• If Y is small, both mechanisms are 2nd order
  (rate of D is inversely related to X)If Y
  is large, both are 1st order in M0, 0-order in
  Y

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Interchange Mechanism

• D and I mechanisms have similar rate laws
• Dissociation Interchange
• ML5X ? ML5 X k1, k-1 ML5X Y ?
  ML5X.Y k1, k1ML5 Y ? ML5Y k2 ML5X.Y ?
  ML5Y X k2 RDS
• If Y is small, both mechanisms are 2nd order
  (and rate of D mechanism is inversely related to
  X)
• If Y is large, both are 1st order in M0,
  0-order in Y

 
 
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Association (A) Mechanism

• ML5X Y ? ML5XY k1, k-1ML5XY ? ML5Y
  X k2
• 1st reaction results in an increased coordination
  number. 2nd reaction is faster
• Rate law is always 2nd order, regardless of Y
• Very few examples known with detectable
  intermediate

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Factors affecting rate

• Most octahedral reactions have dissociative
  character, square pyramid intermediate
• Oxidation state of the metal High oxidation
  state results in slow ligand exchangeNa(H2O)6
  gt Mg(H2O)62 gt Al(H2O)63
• Metal Ionic radius Small ionic radius results
  in slow ligand exchange (for hard metal
  ions)Sr(H2O)62 gt Ca(H2O)62 gt Mg(H2O)62
• For transition metals, Rates decrease down a
  group Fe2 gt Ru2 gt Os2 due to stronger
  M-L bonding

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Dissociation Mechanism
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Evidence Stabilization Energy and rate of H2O
exchange.
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Entering Group Effects
Small incoming ligand effect D or Id mechanism
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Entering Group Effects
Close Id mechanism
Not close Ia mechanism
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Activation Parameters
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RuII vs. RuIII substitution
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Conjugate Base Mechanism
Conjugate base mechanism complexes with
NH3-like or H2O ligands, lose H, ligand trans
to deprotonated ligand is more likely to be
lost.
Co(NH3)5X2 OH- ? Co(NH3)4(NH2)X H2O
(equil) Co(NH3)4(NH2)X ? Co(NH3)4(NH2)2
X- (slow) Co(NH3)4(NH2)2 H2O ?
Co(NH3)5H2O2 (fast)
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Conjugate Base Mechanism
Conjugate base mechanism complexes with NR3 or
H2O ligands, lose H, ligand trans to
deprotonated ligand is more likely to be lost.
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Reaction Modeling using Excel Programming
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Square planar reactions

• Associative or Ia mechanisms, square pyramid
  intermediate
• Pt2 is a soft acid. For the substitution
  reaction trans-PtL2Cl2 Y ? trans-PtL2ClY
  Cl in CH3OHligand will affect reaction
  ratePR3gtCNgtSCNgtIgtBrgtN3gtNO2gtpygtNH3ClgtCH3
  OH
• Leaving group (X) also has effect on rate hard
  ligands are lost easily (NO3, Cl) soft ligands
  with ? electron density are not (CN, NO2)

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Trans effect

• In square planar Pt(II) compounds, ligands trans
  to Cl are more easily replaced than others such
  as ammonia
• Cl has a stronger trans effect than ammonia (but
  Cl is a more labile ligand than NH3)
• CN CO gt PH3 gt NO2 gt I gt Br gt Cl gt NH3 gt
  OH gt H2O
• Pt(NH3)42 2 Cl ? PtCl42 2 NH3
• Sigma bonding - if Pt-T is strong, Pt-X is weaker
  (ligands share metal d-orbitals in sigma bonds)
• Pi bonding - strong pi-acceptor ligands weaken
  P-X bond
• Predictions not exact

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Trans Effect
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Trans Effect First steps random loss of py or
NH3
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Trans Effect
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Electron Transfer Reactions
Inner vs. Outer Sphere Electron Transfer
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Outer Sphere Electron Transfer Reactions
Rates Vary Greatly Despite Same Mechanism
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Nature of Outer Sphere Activation Barrier
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Inner Sphere Electron Transfer
Co(NH3)5Cl2 Cr(H2O)62 ? (NH3)5Co-Cl-Cr(H2O)54
H2O Co(III)
Cr(II)
Co(III) Cr(II)
(NH3)5Co-Cl-Cr(H2O)54? (NH3)5Co-Cl-Cr(H2O)54 Co
(III) Cr(II)
Co(II) Cr(III)
H2O (NH3)5Co-Cl-Cr(H2O)54? (NH3)5Co(H2O)2
(Cl)Cr(H2O)52
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Inner Sphere Electron Transfer
Co(NH3)5Cl2 Cr(H2O)62 ? (NH3)5Co-Cl-Cr(H2O)54
H2O Co(III)
Cr(II)
Co(III) Cr(II)
(NH3)5Co-Cl-Cr(H2O)54? (NH3)5Co-Cl-Cr(H2O)54 Co
(III) Cr(II)
Co(II) Cr(III)
H2O (NH3)5Co-Cl-Cr(H2O)54? (NH3)5Co(H2O)2
(Cl)Cr(H2O)52
Nature of Activation Energy Key Evidence for
Inner Sphere Mechanism
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Example
CoII(CN)53- CoIII(NH3)5X2 ? Products
Those with bridging ligands give product
Co(CN)5X2.